Not applicable.
The present invention generally relates to electromagnetic or EMI shielding. More specifically, the invention relates to an EMI grounding clip for maintaining a grounding path between separable portions of electronic device enclosures.
In the field of electronics, radio frequency (RF) and electromagnetic interference (EMI) are important factors to consider in product designs. On the one hand, EMI waves from external sources have the potential to affect the operation of components inside an electronics package. On the other hand, the FCC and other regulatory agencies impose limits on the frequency and levels of EMI signals that may emanate outwards from the inside of an electronics product. One way to control EMI problems is to package electronics products in an appropriately shielded enclosure. A conventional technique for providing such shielding is to surround internal components with a continuously grounded enclosure. Unfortunately, assembly and service constraints prohibit housing products in a one piece enclosure. Consequently, electronics housings are usually comprised of separate pieces that are fastened together to create a uniform enclosure.
To prevent the EMI problems discussed above, the various pieces of an electronics enclosure must be electrically conductive and coupled to one other. By coupling all pieces of the cover to one another, the entire enclosure can be effectively coupled to ground to form an EMI barrier around the electrical components. Under normal circumstances, simply fastening the pieces of a cover together is not sufficient to ensure electrical conductivity over the anticipated range of operating temperatures and conditions. Internal and external temperature variations may cause some portions of the enclosure to expand or contract at different rates, thereby producing gaps or increasing electrical resistance between pieces of an enclosure. To eliminate this problem, conductive gaskets, clips, or springs may be placed between pieces of an enclosure to maintain electrical conductivity between the pieces. EMI gaskets, clips, or springs are typically compressible and are designed to deflect with enough spring force to always maintain contact with the conductive surfaces of mating pieces of an enclosure.
The use of these conductive EMI devices is well known to those skilled in the art. Representative examples of conventional EMI clips are illustrated and described in U.S. Pat. Nos. 4,554,400 and 4,640,979, both to Schmalz. These EMI clips provide the necessary grounding to ensure RF shielding of electronic devices and, as with many conventional EMI clips, are installed by inserting a portion or portions of the clip into holes in the mounting surface. Conventional EMI clips such as these are therefore retained with tabs that are bent around a mounting hole or with an interference fit between the clip and the mounting hole. There are no other retaining means on these conventional clips. Thus, with these mounting configurations, a common problem is that the clips may become detached and fall out of their mounting locations. This problem is exacerbated by the fact that many conventional EMI clips are low compression clips that are very flexible and very easily deflected. Such characteristics are desirable to permit easier installation of enclosure panels, but also make it much easier for the clips to be pulled out of their mounting locations. Loose EMI clips are naturally a problem in electronics devices because of the potential damage they may cause by electrically coupling and shorting components and electrical traces inside the unit.
To combat this problem, other conventional retaining methods have been used, such as the use of adhesive-backed EMI clips to secure the clips in place. This solution is effective as long as the adhesive retains its bonding power. It is widely recognized, however, that adhesives have the propensity to deteriorate over time and therefore do not provide an effective long-term solution. Another solution has been to combine a series of EMI clips into a single chain of grounding fingers as shown in U.S. Pat. No. 5,917,147 to Lewis. This retaining method presumes that it is more difficult to displace or loosen a large array of grounding clips than smaller, individual clips. While this presumption is not without merit, the large array also requires that there is enough mounting space available to install the chain of grounding fingers. However, given the wide assortment of mounting and spacing configurations in electronics enclosures, it may be desirable to use individual grounding clips instead of such a large array.
Other improved methods of retaining EMI clips are described in U.S. Pat. Nos. 4,803,306 to Malmquist and 5,532,428 to Radloff et al, each of which provide more robust retaining schemes than the methods heretofore described. Images from the former of these two patents (Malnquist) are reproduced in FIG. 1, while images of the latter (Radloff) are reproduced in FIG. 2. The Malmquist clip comprises a U-shaped attaching portion 12 with a retaining notch 18 formed on each side of the U-shaped portion 12. These retaining notches 18 are configured to keep the EMI clip securely in place once the U-shaped portion 12 is inserted into a properly shaped mounting hole 32 in a retaining panel. One distinct disadvantage to this EMI clip is the depth D occupied on the insertion side of the EMI clip. The retaining notches 18 on the U-shaped portion 12 increase the volume requirements for this particular solution. Consequently, additional clearance must be designed into the electronics enclosure so as to prevent interference with the EMI clip.
By way of comparison, the retaining scheme used by Radloff shown in FIG. 2 also occupies additional space on the backside of the mounting surface. In this particular solution, the EMI clip 10 comprises end portions 20 that are substantially wider (dimension L) than the center portion (dimension W) of the clip. The EMI clip is installed into a mounting surface by inserting the end portions 20 through a slot 36 in the mounting surface one end portion at a time. The enlarged end portions 20 are captively held in place by a lance structure 34 that forms a cavity on the backside of the mounting surface in which the end portions 20 reside. When a mating cover is attached to the mounting surface, the EMI clip is compressed such that most of the clip is deflected into the volume between the lance structure 34 and the mounting surface. Unfortunately, this design, as with the Malmquist design, occupies a large volume of space behind the mounting surface (as denoted by dimension D).
Despite the effectiveness of these prior art captivation techniques, each occupies a large volume of space behind the mounting surface. Given the compact nature of electronics packaging, it is desirable, therefore, to provide a robust, low-profile means of captively retaining an EMI clip onto a mounting surface. The novel technique would advantageously decrease the amount of space required to hold the EMI clip in place, thereby permitting lower profile electronics enclosures.
The problems noted above are solved in large part by a novel, low-profile, captive retaining scheme for an EMI clip. The EMI clip is preferably an EMI shielding device comprised of an electrically conductive material and further comprising an elastically deformable body portion capable of applying a contact force between a top compressing surface and a planar bottom mounting surface. The EMI clip includes more than one enlarged, planar end portions that may each be installed in separate apertures in the mounting surface to captively retain the EMI shielding device on the mounting surface. When an installed EMI shielding device is compressed between the compressing and mounting surfaces, the deformable body portion of the EMI shielding device lies flat between the compressing and mounting surfaces. The enlarged end portions lie substantially flat on the side of the mounting surface opposite the side facing the compressing surface.
The EMI shielding device includes a narrow, center, contact portion that bows upwards and inwards from acute bends at the outer ends of the clip. At the top of the contact portion is an obtuse bend configured to bend elastically from some static angle larger than 90 degrees to an angle of 180. The clip also includes flat planar surfaces that ramp upwards from the acute bends towards the common obtuse bend. The enlarged end portions extend flatly inward from the side of the acute bend opposite the planar surfaces. The preferred embodiment of the EMI clip comprises two enlarged, planar end portions. The acute bends on the EMI clip are also elastically deformable to permit compression of the EMI shielding device between the mounting and compressing surfaces and also to permit extension of the EMI shielding device during installation of the EMI shielding device onto the mounting surface.
The preferred EMI clip works in conjunction with a tapered mounting hole pattern in the mounting surface to create an EMI clip retaining system. The planar mounting surface comprises mounting holes through which the enlarged end portions of the EMI clip are inserted such that the narrow center portion of the EMI clip resides on a top side of the mounting surface and the enlarged end portions of the EMI clip reside on the opposite, bottom side of the mounting surface. Once the EMI clip is inserted onto the mounting surface, the EMI clip is captively retained by the tapered shape of the mounting holes and also by the elastic forces within the EMI clip. The mounting surface holes comprise an aperture corresponding to each enlarged end of the EMI clip. The aperture shapes are preferably mirrored about an axis centered between the apertures. Each aperture in the mounting hole pattern comprises a narrow inner width that is wider than the width of the narrow center portion of the EMI clip but narrower than the width of the enlarged end portions of the EMI clip. The aperture also includes a wider outer width that is wider than the width of the enlarged end portions of the EMI clip. The sides of the aperture are tapered and transition from the narrow inner width to the wider outer width.
When the EMI clip is installed onto the mounting surface, the EMI clip is elastically stretched to insert the enlarged ends of the EMI clip through the wider outer width of the corresponding mounting hole apertures. Once inserted, the enlarged portions of the EMI clip contract to a position under the narrow inner width of the corresponding mounting hole apertures thereby captively retaining the EMI clip. The mounting hole apertures are positioned so that the narrow inner widths of the apertures are approximately the same distance apart as are the insides of the acute bends in the EMI clip. By the same token, the wider outer widths of the apertures are not so far apart as to induce plastic deformation of the EMI clip as it is stretched during installation. Alternatively, the mounting hole apertures may be positioned so that the narrow inner widths of the apertures are a larger distance apart than the insides of the acute bends in the EMI clip so as to create an interference contact between the EMI clip and the mounting surface. The preferred mounting surface thickness is approximately twice the thickness of the radii of the acute bends in the EMI clip.